Open your block element in MCreator
Go to the block’s model settings
Set the block model to a custom model instead of a full cube
Import or create a custom model with the size you want
Adjust the model’s dimensions in Blockbench or your 3D editor
Save the model and reimport it into MCreator
Set the block’s bounding box to match the custom shape
Disable full cube rendering if needed
Set the block to non-opaque if required
Adjust collision box settings if the block should not use a full 16x16x16 shape
Test the block in the workspace
Export and run the mod to verify the size in-game